Enhancing the softness of paper products such as tissue and toweling is desirable. Softness is the tactile sensation a user perceives as the user holds, rubs or crumples a particular paper product. This tactile sensation is provided by a combination of several physical properties including the bulk, stiffness and stretchability of the paper.
Creping, a process which is well known in the art, is a means of mechanically foreshortening a fibrous structure in the machine direction in order to enhance the softness, bulk and stretchability of the paper. Creping is generally accomplished with a flexible blade, known as a creping blade, which is placed against a drying surface such as a Yankee dryer. The fibrous structure adheres to the Yankee dryer as it contacts the dryer surface. The web travels along the surface of the Yankee dryer until it is removed by the creping blade. The degree to which the fibrous structure adheres to the Yankee dryer prior to creping is a key factor in determining the degree of softness, bulk, and stretchability exhibited by the fibrous structure after creping.
The level of adhesion of the fibrous structure to the Yankee surface is critical as it relates to the drying of the web. Higher levels of adhesion in combination with relatively low levels of coating build-up on the Yankee dryer surface permit better heat transfer. “Coating build-up” refers to the accumulation of film which builds up on the surface of the Yankee dryer after repeated adhesion/removal creping cycles. The coating build-up results from creping aids applied to the Yankee drum and from materials transferred out of the fibrous structure onto the surface of the Yankee dryer during the drying process (i.e.; hemicelluloses, fines and fiber fragments, wet end chemical additives, and the like). This improved heat transfer enables the web to dry faster, thus allowing the operation to run at higher speeds. Creping aids are preferably applied to the surface of the Yankee dryer to further facilitate the adhesion/creping process.
The level of adhesion of the fibrous structure to the Yankee surface is also important as it relates to the control of the web in its travel from the creping blade to the reel of the paper machine (i.e.; sheet control). Fibrous structures which are insufficiently adhered to the surface of the Yankee dryer are generally difficult to control and often result in quality problems at the reel such as wrinkling, fold-overs and weaved edges. Poor dry end sheet control affects the reliability of the entire papermaking process and subsequent converting operation.
It is important that the creping aid allow for a proper balance between adhesion of the fibrous structure to the drying surface and the release of the fibrous structure at the creping blade. Historically, one of the difficulties encountered with the use of creping aids has been a tendency for the creping aid to form a bond between the fibrous structure and the drying surface at the point of creping such that the fibrous structure does not properly release from the drying surface. This results in portions of the fibrous structure remaining adhered to the surface thus causing defects in the fibrous structure or causing the fibrous structure web to break. One such defect familiar to those of ordinary skill in the art is known as creping blade pickout. Creping blade pickout causes holes in the fibrous structure and increased coating related sheet breaks on the paper machine.
The maintenance of this critical balance has resulted in much development in the area of creping aids. Glues or adhesives such as cationic starches, hemicelluloses, and polyvinyl alcohols are regularly used to increase adhesion. The use of cationic polymeric resins is also well known. For examples, please see U.S. Pat. No. 4,501,640 issued to Soerens on Feb. 26, 1985; U.S. Pat. No. 5,187,219 issued to Furman, Jr. on Feb. 16, 1993; U.S. Pat. No. 5,494,554 issued to Edwards et al. on Feb. 27, 1996; U.S. Pat. No. 5,944,954 issued to Vinson et al. on Aug. 31, 1999; U.S. Pat. No. 5,942,085 issued to Neal et al. on Aug. 24, 1999; U.S. Pat. No. 6,048,938 issued to Neal et al. on Apr. 11, 2000; and U.S. Pat. No. 6,187,138 issued to Neal et al. on Feb. 13, 2001.
Process developments which deliver these components in separate spray boom applications have also been made as demonstrated in U.S. Pat. No. 5,865,950 issued to Vinson et al. on Feb. 2, 1999. Multiple spray booms are also used when both cationic and anionic materials are used in the creping step. The separate delivery systems have been developed to avoid precipitation of the resins in the delivery systems and spray booms. Newer paper-making machines are equipped with a “glue containment box” installed to control over-spray from the oscillating spray header. Glue over-spray has been identified as a problem with respect to maintaining a clean environment around the machine. However, it has been determined that the over-spray captured within the glue containment box resulted in precipitation of the cationic/anionic polymers, resulting in contamination and eventual plugging of the glue containment box.
Unfortunately, while a number of adhesives, including these examples have been disclosed and are available, no single adhesive or adhesive blend has provided a satisfactory combination of adhesion and sheet release, which do not precipitate in the delivery systems or the glue containment box.